1. Field of the Invention
The present invention in general concerns the field of magnetic resonance imaging as used in medicine for examination of patients. The present invention is in particular concerned with the planning and implementation of measurement sequences that allow the hardware used in a magnetic resonance apparatus to be utilized as optimally as possible. Furthermore, the invention concerns a magnetic resonance apparatus and a computer program with which such a method can be implemented.
2. Description of the Prior Art
Magnetic resonance imaging (MR imaging), also known as magnetic resonance tomography (MRT), is based on the physical phenomenon of nuclear magnetic resonance. In this examination modality a subject is exposed to a strong, constant magnetic field. The nuclear spins of the atoms in the subject, which were previously randomly oriented, thereby align. Radio-frequency energy can now excite these “ordered” nuclear spins to specific resonant oscillations. These nuclear magnetic resonances generate the actual measurement signal which is acquired by means of suitable reception coils. Through the use of inhomogeneous magnetic fields (also called gradient fields) that are generated by gradient coils, the measurement signal can be spatially coded with regard to every spatial direction, which is generally designated as “spatial coding”.
The acquisition of the data in MR imaging occurs in k-space (frequency space). The MR image in the image domain is linked with the MR data in k-space by means of Fourier transformation. The spatial coding of the subject which spans k-space occurs by means of gradients in all three spatial directions. Differentiation is made among the slice selection gradient (establishes an acquisition slice in the subject, often the z-axis), the frequency coding gradient (establishes a direction in the slice, often the x-axis) and the phase coding gradient (determines the second dimension within the slice, often the y-axis). A slice is thus initially selectively excited, for example in the z-direction. The coding of the spatial information in the slice ensues via a temporally defined radiation of gradient fields orthogonal to the slice before and during the acquisition of the magnetic resonances, thus in this example by gradient fields that are generated by the gradient coils in the x-direction and y-direction. This coding of the spatial information is also designated as phase and frequency coding.
The entirety of the temporal sequence of the RF pulses and the gradient fields for excitation of the nuclear spins in the image volume to be measured, for signal generation and for spatial coding is known as a measurement sequence. Among other things, the measurement sequence establishes the spatial and temporal characteristic with which k-space is scanned and thereby determines spatial properties of the acquired image (for example extent of the shown region and resolution of the image), or the contrast with which the different tissue types in the image are shown.
It has conventionally been typical that a measurement sequence is completely predetermined by a computerized sequence programmer, with a user allowed only the variation of a specific few variable user parameters within narrow limits. The procedure is advisable since not every conceivable measurement sequence can be realized in a magnetic resonance apparatus due to hardware-dependent limitations. For the user to be able to devise the measurement sequence, the user would have to precisely know the hardware properties of a magnetic resonance apparatus and take these into account in the programming of the measurement sequence together with the variable user parameters so that a measurement sequence that can actually be executed is available at the end. This makes manual sequence programming as well the customizing of a measurement sequence to various systems complicated.
Moreover, the entire possible variation range of the variable user parameters often can not be made available to a user, since the user parameters themselves often depend on one another and on the employed hardware in a complicated manner. For example in order to allow the entire possible parameter space to be selected, all of these complex dependencies of the user parameters would have to be taken into account, which is why it is often simpler to limit the parameter space in advance. For example, the repetition time and the slice thickness depend on one another in a hardware-specific manner. Both of these are user parameters that are often set by a user in a measurement sequence. For this dependency is to be accounted for with generality in the selection of the user parameters would require a large effort and would be dependent on the underlying hardware.
In spite of these often-complex associations between hardware-dependent limitations and adjustable user parameters, a certain degree of freedom still remains in the concrete realization of the measurement sequence after a special selection of user parameters by the user. This means that a sequence programmer also still has latitude in the realization of the measurement sequence after specification of the user parameters by the user and according to the requirements of the specific hardware limitations; thus the measurement sequence can still be realized in different ways, with the different ways nevertheless leading to an essentially identical image result. In general, however, they will still differ with regard to other variables such as, for example, the total duration of the measurement or the loading stressing of individual components of the magnetic resonance apparatus (for example of the gradient coils). These freedoms represent a complication for the sequence programmer since he or she must ultimately make a specific selection with the realization that for the most part, it will not be the optimal selection due to the complex correlations of the user parameters.
Some methods are known with which parts of a measurement sequence can at least be improved. Before describing these methods, the basic concept that underlies many of these methods will be explained.
In recent times it has proven to be advantageous to consider measurement sequences for magnetic resonance imaging as a series of time slices that respectively belong to different types. A first time slice type is thereby the transmission type that is characterized by the presence of in that an RF excitation pulse that is radiated (usually during activation of gradient fields) in time slices of this type. A second type is the acquisition type, characterized by the measurement signal of excited nuclear spins being acquired in time slices of this type. A third time slice type is the warp type, wherein no transmission or acquisition activity exists but gradient fields are radiated (activated) in order to effect a phase coding of the nuclear spins or in order to impress a specific flow coding or diffusion coding on the nuclear spins. The consideration of a measurement sequence as a series of time slices of different types has proven to be advantageous since each type exhibits specific properties for which optimization strategies have been developed.
For example, a method with which dead times in MR pulse sequences can be minimized is described in U.S. Pat. No. 5,512,825. The dead times (dead periods) described therein correspond to the time slices of the warp type described above that exist between time slices of the transmission type and the acquisition type.
In contrast to this, DE 102 14 736 A1 describes a method with which a sampling path within the k-matrix can be calculated under given boundary conditions, wherein the gradient current curves are determined that lead to a sampling along the previously-calculated sampling path given application to the corresponding gradient coils using an analog-digital converter. Boundary conditions that can be taken into account in the calculation of the sampling path are, for example: the maximum load of the gradient amplifier given arbitrary rotation of the k-space matrix, the spatial position of the k-space matrix to be sampled in the subject to be examined, the arrangement of the measurement points in the k-space matrix to be sampled, the sequence type of the sampling, the departure and arrival speed of each measurement point of the k-space matrix, the sequence (order) in which the measurement points of the k-space matrix should be sampled, the avoidance of nerve stimulations of the subject to be examined, and the minimization of the sampling time, the minimization of the slew rate during the sampling.
U.S. Pat. No. 6,636,038 describes a method for controlling a pulse sequence for a nuclear magnetic resonance tomography system in which a control data set for gradient fields, RF pulses and sampling pulses is calculated during the run time of the pulse sequence. The pulse sequence is considered as a series of the time slices cited above. Furthermore, given a specific form of a trapezoidal dephaser gradient pulse train, an optimized trapezium shape of the gradient coil current for a gradient coil is determined such that the gradient coil system is utilized in an improved manner.
With the methods presented here a sequence programmer can be supported to the extent that parts of a measurement sequence are automatically optimized without the sequence programmer having to explicitly specify these parts. After the implementation of those aids, however, the problem also remains that large parts of a measurement sequence must still be manually established. This establishment, which must be made by the sequence programmer before execution of a measurement sequence, often represents a non-optimal compromise between flexibility of the measurement sequence and the hardware-specific optimization of the measurement sequence.